This invention relates to apparatus for potentiometric electrochemical measurements employing ion selective probe systems such as glass pH electrode systems or other ion-selective electrode systems and ISFET pH devices. It is particularly useful in making pH measurements with these probe systems in solutions of low electrolytic conductivity, such as high purity water.
A typical pH measuring system, as found in the prior art, is shown in U.S. Pat. No. 4,189,367, issued to the present inventors on Feb. 19, 1980. That system utilizes an operational amplifier to drive the reference electrode through the low impedence output of the amplifier to maintain the pH electrode, connected to the amplifiers high input impedance summing junction, at signal common voltage. Such a measurement approach is entirely adequate for solutions with an electrolytic conductivity greater than 1 micromho. However, when such systems are used for pH measurements of earth grounded solutions of high purity water (less than 0.1 micromho), such as are found in electric power plants, it has been observed that they are subject to noise pickup and, also, to an offset or shift in the pH reading. It has further been discovered that the major source of error in high purity water measurements is caused by parasitic leakage currents flowing from the grounded solution through the reference electrode when said electrode is connected to a low impedance point in the electronic circuit. These leakage currents then flow through the low impedance path to the common or ground terminal of the power supply, through the power transformer, and through leakage on printed circuit cards in the system to produce, as a result, a leakage path between the circuit common and the AC power line. The instantaneous effects of the leakage currents which flow through this path are produced in direct proportion to the electrolytic resistance of the solution at the reference electrode/solution interface. The presence of parasitic AC currents in this path produces noise in the measurement and parasitic DC currents produce a shift in output, both are directly observed as errors in the pH reading. Longer term integration of DC leakage currents and electrode rectified AC currents will result in a drift of electrode output with a commensurate drift in instrument accuracy.
It is, therefore, an object of this invention to provide a potentiometric electrochemical measuring system for use with grounded low conductivity solutions, such a high purity water, which will avoid noise problems due to spurious AC currents and will avoid level shifts due to spurious DC currents.
It is a further object of this invention to provide a measuring circuit which will preclude the deleterious and permanent shifts in the output voltage of the reference electrode as a result of spurious AC and DC ground loop currents.